Heterogeneity of the pregnancy-specific beta 1-glycoprotein (SP1)

Distribution of SP1, immunoreactivity among different plasma proteins: real molecular heterogeneity or adsorption of SP1-beta to other plasma proteins?

Three SP1-containing factors from pooled term pregnancy sera were subjected to crossed immunoelectrophoresis. New patterns as far as electrophoretic mobilities and shapes of the immunoprecipitates were revealed. The appearance of an additional anodic radioimmunoassayable activity in agarose electrophoresis of mixed SP1-alpha and SP1-beta suggested a binding capacity of SP1-alpha for SP1-beta determinants. In the serum of a single patient at the third trimester of pregnancy we also found two SP1 variants, possessing little radioimmunological reactivity and with crossed immunoelectrophoretic characteristics quite different from those of the 'usual' alpha and beta SP1 forms. These results suggest that, in this particular case, the overall SP1 production cannot be evaluated by competitive binding assay and, that in general, SP1 is a complex antigen the heterogeneity of which can be determined following adsorption of some beta epitopes to another serum protein.

Characterization of cDNAs of the human pregnancy-specific beta 1-glycoprotein family, a new subfamily of the immunoglobulin gene superfamily

Three highly homologous cDNAs encoding human pregnancy-specific beta 1-glycoprotein (SP1) were isolated from a human placental cDNA library. These cDNAs share greater than 90% nucleotide homology in their coding sequences, and greater than 79% of the encoded amino acids are homologous. Proteins encoded by these cDNAs are very similar to members of the carcinoembryonic antigen family and contain repeating domains, conserved disulfide bridges, and beta-sheet structure typical of the immunoglobulin gene superfamily. However, the high degree of sequence homology and relatively lesser degree of glycosylation among the SP1 proteins suggest that they exist as a unique family instead of being members of the CEA family. Both soluble and potentially membrane-bound forms of SP1 proteins were present in the placenta. Northern blot analysis using specific probes confirmed the expression of multiple mRNA species in human term placenta.

Expression of the pregnancy-specific beta 1-glycoprotein genes in human testis

Northern blot analysis with placental pregnancy-specific beta 1-glycoprotein (SP1) cDNA probe showed the presence of SP1 mRNAs in human testis. Presence of translational products of the mRNAs was demonstrated by Western blot analysis with anti-human SP1 antibodies albeit difference in mobilities between the testis and placental proteins was apparent. Screening of human testis cDNA library with placental SP1 probe yielded 4 groups of positive clones. Two groups were identical to human placental SP1 cDNAs previously reported. The other 2 groups consisted of cDNA of incompletely processed mRNAs. These 2 groups were present in high abundance. Sequence analysis suggested that the cDNAs were products of different genes.

Characterization of cDNA encoding human pregnancy-specific beta 1-glycoprotein from placenta and extraplacental tissues and their comparison with carcinoembryonic antigen

Human pregnancy-specific beta 1-glycoprotein (SP1) is one of the early pregnancy proteins produced in large quantity by the placenta during pregnancy. The nucleotide sequence of a cDNA encoding human placental SP1 isolated from a term placental cDNA library was determined. This cDNA contains 1,958 nucleotides with a 5' noncoding sequence of 73 bp, a sequence of 1,257 bp encoding a protein of 419 amino acids with calculated molecular mass of 47.2 kD, a TGA stop codon, and 625 bp of 3' noncoding sequence. Two internal repeat domains, each of 93 amino acids, can be identified within the coding sequence. The positions of two cysteine residues within each repeat are conserved. A cDNA of 489 bp identical to the sequence from nucleotides 422-910 of placental SP1 cDNA was isolated from a human testis cDNA library. Screening of a HeLa cell library with SP1 cDNA probe yielded 10 positive clones. Sequence determination of one of the cloned cDNA inserts revealed a partial cDNA of 773 bp which is 94% homologous at the nucleotide level and 88% homologous at the amino acid level to the placental SP1 cDNA. These observations confirm the observation that some of the SP1 genes might be expressed in extraplacental tissues. Searching through the GenBank/EMBL database revealed 74-80% homology between SP1 cDNA with human carcinoembryonic antigen (CEA) cDNA. The multiple genes of SP1 as a subfamily of the immunoglobulin supergene family is implicated.

Oligomerization of pregnancy-specific beta 1-glycoprotein (SP1) at physiologic pH and ionic strength

Highly purified pregnancy-specific beta 1-glycoprotein (SP1) migrated in gel electrophoresis as a homogeneous species and behaved as a single species in 6 mol/l guanidinium chloride (GdmCl), both in the ultracentrifuge and HPLC. At physiologic pH and ionic strength, in the absence of GdmCl, SP1 existed in the form of oligomers of apparent molecular weights of 40 000 to greater than 300 000. The specific activity of these oligomers varied over a 5-fold range. Electrophoretic mobility also varied among SP1 oligomers, with increasing (alpha-like) mobility shown by oligomers of increasing molecular size. Oligomerization may explain some or all of the reports of SP1 heterogeneity.